The response above is incorrect.
I'm new to ESXi 6.5 and I couldn't find any helpful instructions. Believe me, the process in the web client is NOT intuitive. Hopefully my comment will help some poor soul like me who just wants a quick answer to this simple question.
Here is how I create a VM using a VMDK that I converted from a physical laptop using the Standalone Converter.
Before creating the VM you must "upload" the VMDK to one of your LUNs. To do this, in ESXi web client navigate to Storage, highlight the LUN and Browse Files. Click Upload Files and navigate to your attached USB drive or wherever the VMDK is.
Then follow the normal steps to create a new VM. When you get to step 7 Customize Hardware, delete the "New Hard Disk" which has probably has a 32GB drive by default. Click Add New Device and select Existing Hard Disk. From there you can select where you've uploaded the VMDK noted above.
Go to the datastore on which the VM is stored. Right click on the datastore and click Register VM, Select the VMK file associated with VMDK and register the VM in the Inventory.
Here is the detailed procedure.
Just to clarify here; I was just working on this with 6.7 and registering the VM via datastore browser will not work unless the .vmx is present and associated with the .vmdk. If this is a net new virtual machine, the correct procedure is to upload the .vmdk to the desired datastore and create the virtual machine utilizing the uploaded .vmdk as the existing hard disk. The .vmdk will provide the Guest OS and associated data, but without the .vmx there are no defined configuration parameters (e.g. CPU, Memory) which is why the existing disk needs to be associated via a new VM build.
Edit: this is assuming the .vmdk is formatted for ESXi. VMware vCenter Converter will need to be used if the .vmdk is formatted for a VMware Hosted product such as Workstation or Fusion.